Exploring the BiFeO₃-PbTiO₃-SrTiO₃ Ternary System to Obtain Good Piezoelectrical Properties at Low and High Temperatures

In this work, we investigated the piezoelectric properties of BiFeO3-rich (1 - (y + x)) BiFeO3-y PbTiO3-x SrTiO3 (0.1 <= x <= 0.35; 0.1 <= y <= 0.3) bulk piezoceramics, as this system could potentially lead to the development of bulk piezoelectric ceramics that are suitable for high-temperature applications (>200 degrees C). Samples with various levels of PbTiO3 and SrTiO3 were prepared via a conventional solid-state route. X-ray diffraction confirmed a pure perovskite phase for the compositions, which was explored without secondary phases. It was found that the addition of comparable levels of PbTiO3 and SrTiO3 to the BiFeO3 ceramic resulted in higher piezoelectric properties compared to those of the pure BiFeO3 and binary systems. The Curie temperature was significantly reduced by dual doping, with SrTiO3 and PbTiO3 additions resulting in comparable Curie temperature depressions. The locations of the phase boundaries between the cubic, pseudocubic, and rhombohedral crystal structures were determined. The highest piezoelectric properties, including a d(33) value of 250 pC/N at room temperature, were obtained for the samples with the composition x = 0.3, y = 0.25, which was close to the cubic-pseudocubic phase boundary in the phase diagram. The temperature dependence of the piezoelectric properties varied depending on the previous thermal history, yet an appropriate heat treatment resulted in an almost temperature-stable d(33) value. The ceramic with the lowest temperature sensitivity and a high Curie temperature of 350 degrees C was found for x = 0.1, y = 0.2 with a d(33) value of 60 pC/N at RT and 71 pC/N at 300 degrees C (after poling at 60 kV/cm and a stabilizing heat treatment). However, the materials developed were still unsuitable for applications at high temperatures due to a rapidly increasing electrical conductivity with increasing temperature.